Your scenario is similar to Northwind OrderDetails -> Products - > Suppliers. Here is what you can do:
I've added Change event for the Product DropDown in Add Order Details page to execute the following code:
orderdetail.Product = getProductsResult.FirstOrDefault(p => p.ProductID == ${event})
I've added also additional row to the form with read-only TextBox component to show the City of the product supplier for this order detail:
The supplier city TextBox Value expression is @orderdetail?.Product?.Supplier?.City - this will tell Radzen to generate simple expression isntead ${} which will generate bind-Value expression.
The result:
